This invention relates generally to methods of billet casting. Steel is usually produced in a continuous casting process, which involves continuous delivery of molten metal to a caster during a casting campaign. In billet casting, the molten metal is delivered from ladle into a tundish and continuously fed into a mold which simultaneously creates several strands of steel, each strand shaped in a cross section of desired product. Generally, the tundish holds an amount of the molten metal so ladles can be changed without disrupting casting and controls the flow of said molten metal into the mold. After the mold, the strands are taken through guides that take the strands in a curvilinear path and horizontally orient the strands for further processing. The material may be sprayed with a cooling liquid at any point after exiting the mold. After the material has sufficiently cooled and is oriented on the runout table the billets are cut to the desired lengths.
Billet casting may be done in continuous casting the same as slabs with subentry nozzles, but that is generally product dependent and expensive. This method requires significant investment in both the continuous casting equipment and ongoing maintenance. There are also additional characteristics of continuous casting with a subentry nozzle which make it unsuitable for traditional billet casting. There is a need to provide a cost-efficient method of billet casting which produces billets of improved steel quality. Further, billet casting generally employs a mold or molds having a shape of the desired cross sections for the steel products produced. A cost effective method is needed which can eliminate or reduce the amount of waste of casting and improve the quality of a billet.
Disclosed is an efficient method of billet casting which produces billets of improved steel quality without using traditional slab continuous casting. The disclosed method of billet casting comprises the steps of: assembling a billet caster with a shroud extending from a tundish to just above a mold such that the shroud does not contact the molten metal in the mold, delivering the molten metal from a ladle and into the tundish, delivering the molten metal from the tundish through a shroud and to the mold, the shroud inhibiting contact between the molten metal and surrounding atmosphere, casting the molten metal into billets from the mold and cooling the billets below the mold with coolant spray to form cooled billets, and delivering the cooled billets to a runout table to be cut to length. In some examples, the shroud extends between about 1 and 55 mm above the meniscus of the molten metal in the mold. More specifically, the shroud may extend between 1 and 15 mm above the meniscus of the molten metal in the mold.
In methods of billet casting, the shroud comprises a passage for delivering the molten metal to the mold. The passage in the shroud may be tapered from a first shroud end near the tundish to a second shroud end near the mold and above the meniscus in the mold. Further, the passage at the first shroud end may be larger than the passage at the second shroud end and may be tapered. In other examples, the shroud may not be tapered.
In some methods of billet casting, the shroud is formed of refractory material. In particular examples, the refractory material is an alumina-based material. The refractory material may be entire shroud or a portion of the shroud. By example, the refractory material has a thickness of ⅛ inch or more. The refractory material may additionally or alternatively have a variable thickness. Further, the refractory material may be encased by a metal casing. In a particular example, the metal casing has a thickness of 1.5 mm.
In various methods of billet casting, the shroud comprises an upper portion located near the tundish and a lower portion located near the mold, where the upper portion is located above the lower portion. The upper portion may be formed of a material different than the lower portion. By example, the upper portion may comprise a pressed silica outer portion and a zirconia inner portion. One or both portions may be encased by a metal casing, such as previously described. In one example, the upper portion forms a nozzle with a nozzle passage extending from near the tundish to the lower portion. The nozzle passage may comprise a first nozzle end near the tundish and a second nozzle end near the lower portion, where the nozzle passage at the first nozzle end is larger than the nozzle passage at the second nozzle end. By example, the nozzle passage at the first nozzle end may have a diameter of 28.7 mm and the nozzle passage at the second nozzle end may have a diameter of 17.5 mm. Further, a passage of the lower portion may have a larger diameter than the passage at the second nozzle end.
The shroud extends from tundish to just above the meniscus of the molten metal in the mold. The molten metal in the shroud does not come into contact with the surrounding atmosphere. In this way, the steel composition of the molten metal is expected to absorb up to about 85% less oxygen, nitrogen, and other elements and compounds from the surrounding atmosphere. The shroud may extend to any selected height above the meniscus of the molten metal in the mold, and may extend to between 1 mm and 55 mm above the meniscus of the molten metal in the mold or to between 1 mm and 15 mm above the meniscus of the molten metal in the mold.
The method may further comprise assembling a dummy bar adapted to start casting of shrouds in the mold, and after starting casting, allows the billet casting campaign to proceed, or continue once started. The dummy bar may be a solid piece of metal billet stock with the same cross section as the desired end product. The dummy bar is positioned into the mold from below, to start casting. After the dummy bar is in position, molten metal is delivered from the tundish through the shroud and into the mold. The dummy bar then moves through a plurality of rollers to the run out table, allowing a next campaign of casting to begin.
After the dummy bar passes through the casting station, the dummy bar is removed from the newly formed cast strand. This removal may take the form of rolling the dummy bar through a series of separate rollers. The dummy bar may then stay in that position until the start of another casting campaign.